We use unna boots for many people with venous stasis ulcers or just plain edematous, "weepy" legs. Unna boots are NOT, by manufacturing specifications, indicated for compression, however when physicians write, "unna boots to BLE for compression" this is acceptable, and is the most common use in my experience.
If any dressing is placed under an unna boot it would need to be something that can remain intact for the duration the boot is left on (sounds like common sense but sometimes it isn't). We use Xcell AM covered with adaptic if the wound is dry or has any type of eschar, or just plain adaptic. Then, the unna boot is applied as explained by ktrn2b, from the base of the toes to about two finger widths below the popliteal space. Some instructions will direct you to apply the boot in a figure 8 type fashion, however this can be dangerous if another nurse who is not familiar with the process removes this unna boot to apply another and attempts to repeat the figure 8. Any gaps left in the boot will trap the edema and may cause more ulcerations or tissue death.
An important factor to consider is the pt's ABI--we normally will not compress anyone with less than 0.8 or more than 1.2. We normally use a dressing as described above under the unna boot on any ulcers, followed by the unna boot itself with a 75% overlap on each wrap, then kerlix gauze and finally coban. The kerlix will absorb the exudate that the sorbitol in the unna boot pulls away and the coban will keep the compression of the unna boot in place until removed.